This invention relates to an optical instrument and a device manufacturing method. More particularly, the invention concerns an optical instrument suitably usable in an exposure apparatus or a spectroscope, for example, which uses light of a wavelength in an ultraviolet region as a light source and which has a function for preventing contamination of an optical element provided therewithin. Also, the invention concerns a device manufacturing method using such optical instrument.
As regards light sources of optical instruments, recently, shortening of the wavelength of light has been required. Currently, in place of standard ultraviolet rays, deep ultraviolet rays, X-rays and EUV, for example, are used. Generally, the shorter the wavelength is, the larger the optical energy thereof is. For example, photon energies of excimer lasers are 114.1 Kcal/mol (KrF excimer laser of a wavelength 248 nm), 147.2 Kcal/mol (ArF excimer laser of 193 nm), and 180.1 Kcal/mol (F2 laser of 157 nm). As compared therewith, the binding and dissociation energy of molecule is, for example, 84 Kcal/mol (Cxe2x80x94C bond). Namely, photon energies in this wavelength region correspond to binding and dissociation energies of various substances. Thus, when a substance is irradiated with a photon energy, there occurs optical absorption or opto-chemical reaction.
On the basis of such property, light of such wavelength region can be used to process a substance. Also, because the optical characteristic such as absorption or reflection differs with a substance, it can be used for the structure analysis of a substance. Thus, light in such wavelength region is used in a lithographic process, a CVD process, and an etching process, and in various measurement instruments.
In such wavelength region, particularly, a wavelength region not longer than 220 nm, however, oxygen absorbs light. This is because, with the shortening of the wavelength, the photon energy becomes larger and there occurs light absorption by oxygen. In consideration of it, in optical instruments using light of such wavelength region, the light path of an optical system is kept at vacuum or is filled with an inactive gas to prevent absorption by oxygen. The absorption of light is caused not only by oxygen but also by various substances. Further, there may occur decomposition or composition of a substance by any opto-chemical reaction.
Therefore, a substance deposited on an optical element such as a lens, a mirror, a mask or a reticle, for example, may cause light absorption.
Also, a substance produced by an opto-chemical reaction may be deposited on an optical element and may cause deterioration of its optical characteristic. In order to prevent such inconveniences, conventionally, an inactive gas to be supplied is kept at a high purity, or a filter for moving impurities (taking inorganic ion sulfate or ammonia as impurities) is mounted, for example.
Ammonium sulfate which is a typical contaminating substance is produced from sulfate ions and ammonium ions. The source of them may be those originally contained in an ambience gas of the optical instrument or those produced from the surface of a member. Further, it has been reported that, where water vapors are contained in a nitrogen gas ambience, irradiation of ultraviolet rays causes creation of ammonia. Also, it has been reported that an optical element may be contaminated by deposition of silicon oxide caused by an organic silicon compound.
The deterioration of optical characteristics of an optical element by deposition of a substance on its surface becomes more serious as the wavelength of a light source is shortened.
This is because, first, even if a substance deposited on an optical element does not adversely affect the optical characteristic thereof in a wavelength range from a visible region to a standard ultraviolet region, the same substance can absorb light of a shorter wavelength and apply an adverse effect to the optical characteristic.
Further, as the photon energy becomes stronger, an opto-chemical reaction attributable to a substance which is present on a light path may be promoted.
In consideration of the above, when light of a shorter wavelength is used, not only ion sulfate, ammonia and organic silicon compound but also many organic substances to which attentions have not been paid heretofore should be considered as factors for deteriorating the optical characteristic, and appropriate measures should be taken thereto.
From the standpoint of preventing contamination of an optical element, desirably all the impurities in an optical instrument should be removed. Practically, however, there are impurities in a gas from a supply source and, additionally, degassing may occur from a component of an optical instrument or a gas supply unit.
A factor to be considered in practice in relation to contamination of optical elements of an optical instrument due to deposition of impurities is the density or concentration of impurities in each portions surrounding the optical elements, which may cause deposit contamination substances.
Therefore, not only the purity of a gas from a supply source but also matters decomposed from components of the optical instrument or a gas supply line, for example, should be considered from the standpoint of impurities, and it is necessary to design a contamination-free environment. The impurity production due to these factors is not constant. If there occurs deterioration of or defect in a component, the impurity concentration in the optical instrument will increase due to matters decomposed from the component, causing contamination of optical elements.
It is an object of the present invention to provide an optical instrument by which contamination of an optical element due to deposition of impurities can be reduced.
It is another object of the present invention to provide a device manufacturing method using such optical instrument.
There may be various impurities inside an optical instrument. Among them, particularly to those which may be deposited on an optical element to cause deterioration of its optical characteristic, the density or concentration thereof should desirably be defined and the density inside the optical instrument should preferably be monitored and controlled. The impurity density in the ambience and the density of being deposited and accumulated on the surface of an optical element are at a certain proportion, for each substance, and are in a balanced state. Therefore, for suppressing deposition thereof on the surface of an optical element, it is necessary to decrease the impurity density or concentration in the ambience of the optical instrument and also to monitor and control the concentration.
In accordance with an aspect of the present invention, there is provided an optical instrument, comprising: an optical element; and a detector for detecting an impurity concentration in an ambience containing a space surrounding the optical element.
In accordance with another aspect of the present invention, there is provided an optical, instrument, comprising: an optical element; a detector for detecting an impurity concentration in an ambience containing a space surrounding the optical element; and means for producing information of impurity concentration on the basis of an output of said detector.
In accordance with a further aspect of the present invention, there is provided an optical instrument, comprising: an optical element; a detector for detecting an impurity concentration in an ambience containing a space surrounding the optical element; and means for informing an abnormal concentration on the basis of an output of said detector.
In accordance with a yet further aspect of the present invention, there is provided an optical instrument, comprising: an optical element; a detector for detecting an impurity concentration in an ambience containing a space surrounding the optical element; and a controller for controlling said optical element on the basis of an output of said detector.
In these aspects of the present invention, the optical instrument may further comprise means for putting the ambience in a state purged with a gas substantially not absorbing light to be propagated through the optical element.
The gas may comprise a dry air or an inactive gas such as a nitrogen gas and a helium gas.
The light may comprise deep ultraviolet rays having a wavelength not longer than 200 nm.
The inactive gas may comprise a helium gas.
The light may comprise deep ultraviolet rays having a wavelength of about 248 nm.
The optical instrument may further comprise an excimer laser as a light source for producing the light.
The optical instrument may be an exposure apparatus including (i) means for holding one of a mark and a reticle (ii) an illumination optical system for illuminating a pattern of the mask or the reticle with the light, and (iii) means for holding a wafer to be exposed with the pattern. Also, the optical instrument may further comprise a projection optical system for projecting the pattern onto the wafer with use of the light, wherein said projection optical system is provided by (i) refractive elements only, (ii) reflective elements only, or (iii) a combination of refractive and reflective elements.
The detector may have a sensor for detecting a concentration of an organic substance.
The concentration of the organic substance may be controlled so that the total amount of organic substance in a gas inside said optical instrument becomes not greater than 1 xcexcg/m3.
The concentration of the organic substance may be controlled so that each concentration of carboxylic acids, aldehydes, esters, phenols, phtalates, phthalic acids, amines, and amides is kept at 0.01 xcexcg/m3 or less.
In accordance with a still further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: exposing a wafer with a device pattern by use of an optical instrument as recited above; and developing the exposed wafer.